Piezoelectric transducer having good sensitivity over a wide range of temperature and pressure

ABSTRACT

An electroacoustic transducer wherein the sensing elements comprise a radially polarized ceramic cylinder having annular rings attached to the ends of the ceramic cylinder and a cylindrical insert, centered in said cylinder, is sealed by O rings between the insert and the annular rings. A pair of the ceramic cylinders are connected electrically to form an assembly. The assembly is mounted by rubber mounts in a frame which is surrounded by an expanded metal electrostatic shield. The whole assembly is enclosed within a castor oil filled butyl boot. The assembly has good sensitivity over a wide frequency range.

United States Fatent 91 Henriqnez et a1.

11] 3,827,023 1 July 30,1974

The United States of America as represented by the Secretary of the Navy, Washington, DC.

Filed: May 25, 1972 Appl. No.: 256,864

[73] Assignee:

References Cited UNITED STATES PATENTS 10/1952 Massa 340/10 9/1956 Vogel 340/8 UX 4/1965 Horsman et al. 340/10 7/1966 .lunger et al. 340/10 3,263,208 7/1966 Douglas 340/10 X 3,309,653 3/1967 Martin et al. 340/10 3,328,752 6/1967 Sims 340/10 3,509,522 4/1970 Whitlell 340/10 Primary E.raminerSamuel Feinberg Assistant Examiner-H. J. Tudor Attorney, Agent, or Firm-R. S. Sciascia; Arthur L. Branning; Philip Schneider [57] ABSTRACT An electroacoustic transducer wherein the sensing elements comprise a radially polarized ceramic cylinder having annular rings attached to the ends of the ceramic cylinder and a cylindrical insert, centered in said cylinder, is sealed by 0 rings between the insert and the annular rings. A pair of the ceramic cylinders are connected electrically to form an assembly. The assembly is mounted by rubber mounts in a frame which is surrounded by an expanded metal electrostatic shield. The whole assembly is enclosed within a castor oil filled butyl boot. The assembly has good sensitivity over a wide frequency range.

5 Claims, 3 Drawing Figures 30 24 2O 32 IO 18 BACKGROUND OF THE INVENTION This invention is directed to piezoelectric type transducers or hydrophones and more particularly to a transducer which is small, broadband, temperatureand pressure-stable for use as an underwater sound reference standard or as a sound source in wide range applications where the acoustic characteristics must remain stable over long periods of time.

Prior art devices which operate in the frequency range of interest, HZ to 60 kHz, and at hydrostatic pressures from 0 to 3450 kPa had recurring response or sensitivity changes of as much as 4 dB with a temperature change from 30 to 3C. The response characteristics indicate a sensitivity decrease of approximately 0.5 dB per 690 kPa 100 psig) applied hydrostatic pressure at frequencies below 60 kHz. These prior art transducers employ cellular pressure-release materials which has been determined to be the primary cause of the disadvantages described above.

Until now the state of the art in piezoelectric materials and transducer design dictated the use of the pressure-release material to achieve the desired acoustical and electrical characteristics (response, sensitivity, bandwidth, impedance, and directivity). Artificial piezoelectric materials such as barium titanates, lead zirconates, etc. provide an excellent sensing material and have greatly advanced the studies in underwater research, however, special consideration must be given to mounting and assembly techniques.

Many techniques of assembling the piezoelectric sensor have resulted in increased acoustical stability (with pressure and temperature changes) however the present invention further advances the stability with good sensitivity over a much broader bandwith, as disclosed in the following summary.

SUMMARY OF THE INVENTION The transducer of this invention may be used as a reference standard through a wide range of frequency; as a sound source over an also wide range of frequency. It is small, lightweight, broadband, temperatureand pressure-stable. The transducer elements comprise a radially polarized ceramic cylinder having, annular rings attached to the ends of the ceramic cylinder, a cylindrical insert centered in said cylinder which is sealed by 0 rings between the insert and the annular rings. The resulting element when used in a transducer is more rugged with acoustical characteristics that are stable over an extended time period.

OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide a simple, small, lightweight, broadband, rugged, temperatureand pressure-stable transducer suitable for use as a reference standard over extended time periods (over extended periods of continuous submergence).

Another object is to provide an improved sensing element for use in transducers.

Yet another object is to provide an electroacoustic transducer employing annular rings attached to a ceramic element, an insert and mediums for sealing the rings to the insert.

Other objects and advantages of the invention will become apparent by reference to the following description of the drawings and preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a sectional side view of the transducer crystal assembly.

FIG. 2 illustrates a typical free-field voltage sensitivity of the transducer.

FIG. 3 illustrates a typical transmitting voltage response.

DESCRIPTION OF THE PREFERRED EMBODIMENT Now referring to the drawings, FIG. 1, there is shown a crystal assembly for use as a transducer. While primarily for use as an underwater sound receiver in the frequency range of 1 Hz to kHz, the transducer can be used as a sound source in the range of 10 to 70 kHz. The transducer is omnidirectional within 1 dB to 70 kHz in the plane (XY) normal to its longitudinal axis. The directivity in the vertical (XZ) plane is equivalent to that of a 4.0-cm line.

Elements l0 and 12 are connected to form the sensors of the transducer 14. Referring to element 10 which is in section, each of the elements comprises a radially polarized piezoelectric ceramic cylinder 16 [Type III ceramic, MIL-STD 1376 (SHIPS), 21 Dec. 1970] with magnesium end rings 18 bonded to each end of the cylinder 16 by epoxy adhesive 20. A cylindrical insert 22 made of magnesium has annular grooves 24 cut therein to hold 0 ring seals 26. Insert 22 may be made of other materials, but whatever materials are chosen, the insert must be made of a material which gives 22 a resonant frequency greater than the frequency range of the ceramic cylinder 16, for proper response and maximum sensitivity of the device. With the O ring seal 26 in place the cylindrical insert 22 is slipped into the ceramic cylinder 16 and the 0 rings 26 seal against the end rings 18. The inner electrode wire 28 is soldered to the inside of the wall of ceramic cylinder l6 and passes to the exterior through a small exit channel which is sealed with epoxy. Electrode wire 28 is, then channeled into element 12 and again soldered to the inside of the ceramic cylinder and then channeled out for connection to a connector (not described). Outside electrode wire 30 is soldered to the outside of the ceramic cylinder and passes under a rubber mount 32 to element 12 where it is again soldered to the outside. The wire 30 is then passed under mount 32 to a connector (not described). An expanded metal shield 34 in the form of a cylinder serves as a mechanical support for rubber mounts 32, an electrostatic shield and guard for the sensitive elements 10 and 12 which are mounted in natural rubber mounts 32.

The assembly described is then filled with castor oil 36 and covered by butyl rubber boot 38. The castor oil serves as the acoustic medium between the boot 38 and the elements 10 and 12.

' The elements approaches the stability of a capped cylinder without the use of end caps. The O-rings 26 mechanically isolate the insert 22 from the ends of the ceramic cylinder 16 and eliminate the end-flexure resonances that occur in a conventionally capped ceramic cylinder thus reducing the potential bandwidth of the ceramic cylinder. A significant feature is that the rings 26 seal on the end rings 18 and not on the inside wall of the ceramic cylinder 16. This isolates changes in the mechanical properties of the O-ring materials from the piezoelectric material. The elements l0, l2 operate in a semicapped/semi end-exposed mode with the most desirable response characteristics of each basic mode. Thus the elements l0, 12 have the broadband response of an end-exposed cylinder without the need for cellular pressure-release material.

The expanded metal shield 34 serves as both a mechanical mounting structure and an electrostatic shield. The expanded metal tube is a strong, rugged structure, offering maximum protection to the sensor elements l0, 12. The grid density of the shield 34 makes it an effective electrostatic shield, while the metal structure does not cause any acoustical interference within the operational frequency range of the transducer.

Therefore, there has been disclosed a transducer element with a free-field voltage sensitivity that is relatively flat', of broad bandwidth, and stable with temperatures from 3 to 30C and with hydrostatic pressure from 0 to 6950 kPa.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. Various materials can be used for the ceramic cylinder 16, the end rings 18 and the insert but the characteristics will vary from those given.

What is claimed and desired to be secured by letters patent of the United States is:

1. An electroacoustic transducer which comprises:

an elongated hollow cylindrical body comprising;

an elongated hollow cylindrical piezoelectric element electrically responsive to incident pressure;

2. The electroacoustic transducer described in claim 1 wherein said piezoelectric element is radially polarized.

3. The electroacoustic transducer described in claim 1 wherein said isolating means comprises:

a solid cylinder of radius less than the inner radius of said cylindrical piezoelectric element and said end rings;

said cylinder having a resonant frequency greater than the range of said cylindrical piezoelectric ele ment.

4. The electroacoustic transducer described in claim 3 wherein said mounting means are O-rings mounted on each end of said solid cylinder.

5. The electroacoustic transducer described in claim 4 wherein said cylindrical piezoelectric element is mounted in rubber bushings, surrounded by castor oil;

said rubber bushings being supported by expanded cylindrical metal shield means for providing mechanical support to said elements and for providing an electrostatic shield. 

1. An electroacoustic transducer which comprises: an elongated hollow cylindrical body comprising; an elongated hollow cylindrical piezoelectric element electrically responsive to incident pressure; a first end ring being concentrically bonded to a first end of said cylindrical piezoelectric element; a second end ring being concentrically bonded to a second end of said piezoelectric element; isolating means inserted within said elongated hollow cylindrical body for providing acoustical isolation to the inner wall of said cylindrical piezoelectric element; mounting means mounting said isolation means within said elongated hollow cylindrical body; said mounting means cooperating only radially and sealingly between said end rings and each end of said isolating means.
 2. The electroacoustic transducer described in claim 1 wherein said piezoelectric element is radially polarized.
 3. The electroacoustic transducer described in claim 1 wherein said isolating means comprises: a solid cylinder of radius less than the inner radius of said cylindrical piezoelectric element and said end rings; said cylinder having a resonant frequency greater than the range of said cylindrical piezoelectric element.
 4. The electroacoustic transducer described in claim 3 wherein said mounting means are O-rings mounted on each end of said solid cylinder.
 5. The electroacoustic transducer described in claim 4 wherein said cylindrical piezoelectric element is mounted in rubber bushings, surrounded by castor oil; said rubber bushings being supported by expanded cylindrical metal shield means for providing mechanical support to said elements and for providing an electrostatic shield. 